The present invention relates to a semiconductor laser device, a manufacturing method thereof, and a laser bar locking apparatus.
As is in many cases of semiconductor laser devices, a GaAs laser chip 101 is provided with protective films 103, 104 having a same reflectance on light emitting end surfaces 101a, 101b of the GaAs laser chip 101, as shown in FIG. 10. The reference numeral 102 in FIG. 10 denotes an active layer of the GaAs laser chip 101. When the protective films 103, 104 have the same reflectance as stated above, both optical outputs from the light emitting end surface 101a and the light emitting end surface 101b are Po.
In the case where the protective films 103, 104 are structured from Al2O3 and given a refractive index of 1.60, when the. GaAs laser chip 101 is given a refractive index of 3.50, a reflectance of the protective films 103, 104 is changed by changing the film thickness thereof as shown in FIG. 11 (a laser emission wavelength λ=7800 Å).
Regardless of thickness of the protective films 103, 104, the protective films 103, 104 has a reflectance smaller than that of the GaAs laser chip 101. In the above case, when the optical film thickness of the protective films 103, 104 is odd multiples of λ/4, the reflectance of the protective films 103, 104 becomes the smallest. On the other hand, when the optical film thickness of the protective films 103, 104 is integral multiples of λ/2, the reflectance of the protective films 103, 104 becomes the largest and approximates most to the reflectance of the GaAs laser chip 101. This is because the refractive index of the protective films 103, 104 is smaller than the refractive index of the GaAs laser chip 101. It should be noted that the optical film thickness is defined as a film thickness multiplied by a reflectance.
In the case where the refractive index of the protective films. 103, 104 is larger than the refractive index of the GaAs laser chip 1, for example, where Si film is used as the protective film, the reflectance of the Si film becomes larger than that of the GaAs laser chip 101 regardless of the thickness of the Si film. In the above case, when the optical film thickness of the Si film is odd multiples of λ/4, the reflectance of the Si film becomes the largest. On the other hand, when the optical film thickness of the Si film is integral multiples of λ/2, the reflectance of the Si film becomes the smallest and approximates most to the reflectance of the GaAs laser chip 101.
In the case of a semiconductor laser device having a high output laser with an optical output of 20 mW or more for example, as shown in FIG. 12, there is provided a protective film 113 with a reflectance smaller than the reflectance of a laser chip 111 on a front-side light emitting end surface (main emitting face) 111a. Also, there is provided a protective film 114 with a reflectance larger than the reflectance of the laser chip 111 on a rear-side light emitting end surface 111b. Consequently, optical output Pf from the front-side light emitting end surface 111a of the laser chip 111 becomes higher than optical output Pr from the rear-side light emitting end surface 111b of the laser chip 111. For example, the protective film 113 on the light emitting end surface 111a is formed from Al2O3 so as to have a film thickness of approximately 700 to 1,600 Å, and the reflectance thereof is set to be approximately 15% or less. Here, a reference numeral 112 in FIG. 12 denotes an active layer of the laser chip 111.
Also, the protective film 114 on the light emitting end surface 111b; if composed of one layer, cannot attain a sufficiently high reflectance even if the refractive index thereof is larger than that of the laser chip 111. Therefore, the protective film 114 is composed of a plurality of layers. Specifically, the protective film 114 is composed of a first layer 114a to a fifth layer 114e. The first layer 114a and the third layer 114c are Al2O3 films with a thickness of λ/4 (λ: laser emission wavelength). The second layer 114b and the fourth layer 114d are amorphous Si films with a thickness of λ/4. The fifth layer 114e is an Al2O3 film with a thickness of λ/2. Thus, the protective film 114 attains a reflectance of approximately 85% or more.
Following description discusses a conventional manufacturing method of semiconductor laser devices.
First, in a semiconductor laser wafer 100 shown in FIG. 13, a cleavage line 117 is formed by scribe between an electrode 115 on a specified laser chip and an electrode 115 on a laser chip adjacent to the laser chip in direction orthogonal to a light emitting portion (channel) 118. Then, the semiconductor laser wafer 100 is cleaved. This provides a laser bar (a bar of laser chips) 121 from the semiconductor laser wafer 100 as shown in FIG. 14.
Next, the laser bars 121 are set into a laser bar locking apparatus 150 such that the electrode faces of the laser bars 121 are piled, as shown in FIG. 15. The laser bars 121 are also set into the laser bar locking apparatus 150 such that the front-side emitting faces of all the laser bars 121 and the rear-side emitting faces thereof face in the same direction, respectively.
Next, a protective film having a specified reflectance is formed on the light emitting end surface of the laser bar 121 which is locked in the laser bar locking apparatus 150. In this case, a vacuum depositor 170 is generally used as shown in FIG. 16. The vacuum depositor 170 is equipped with a vapor source 172, a rotating holder 173 for holding the above-described laser bar locking apparatus 150, and a crystal oscillator 174 disposed in the vicinity of the rotating holder 173 for monitoring deposition thickness of film, all of which are housed in a chamber 171.
Following description discusses a procedure of forming the protective film.
First, gas in the chamber 171 is exhausted through a duct 175 so as to put the chamber 171 in a vacuum. When a vacuum degree in the chamber 171 reaches a specified value, an deposition material 176 in the vapor source 172 is heated by an electron beam or the like for deposition. Thereby, the deposition material 176 is deposited on one light emitting end surface of the laser bar 121 to form a protective film.
After that, the rotating holder 173 is turned over by 180° rotation, and the deposition material 176 is again heated by an electron beam or the like for deposition. Thereby, the deposition material 176 is deposited on the other light emitting end surface of the laser bar 121 to form a protective film. A formation speed (deposition rate) of forming protective films on the both light emitting end surfaces of the laser bar 121 is so controlled as to be generally constant until completion of deposition. The deposition rate is controlled by a heating temperature, and therefore, the control in the electron beam deposition is performed by intensity of the electron beam. In the case of resistance heating, it is well known that control of the deposition rate is performed by controlling electric current applied to a resistive element. Specifically, when the deposition material is Al2O3, the deposition rate is generally set between several to 30 Å per sec. The deposition for the protective film is performed while film thickness of the protective film is monitored by the crystal oscillator 174. The deposition is terminated when the film thickness of the protective film reaches a specified film thickness.
In the case where a protective film is formed on an end surface of a laser chip by deposition, a partial pressure of oxygen molecules rises immediately after start of deposition, the oxygen molecules being generated from oxide (Al2O3) as a material for the protective film. There is a high possibility that a damage is caused on the end surface of the laser chip since the oxygen molecules collide with or bond to the end surface of the laser chip. Also, the damage is further increased if an active layer of the laser chip or an adjacent layer of the active layer is made of any compositions including aluminum. Therefore, a reliability has not been ensured when the semiconductor laser device manufactured according to the above-stated is so operated as to obtain a high output.
For a solution of such a problem as the above, a laser chip 111 as shown in FIG. 17 has been proposed. A Si thin film 123 of about 20 Å in thickness is deposited on a front-side light emitting end surface 111a of the laser chip 111, and thereafter the protective film 133 is formed on the Si thin film 123. In this case, the Si thin film 123 is first formed, decomposition of which does not generate oxygen during deposition. Therefore, in the state of low partial pressure of oxygen, film formation in the vicinity of the end surface of the laser chip 111 may be performed since immediately after start of deposition. As a result, the above-described damage on the vicinity of the end surface can be advantageously restrained and reliability in high output operation is fully ensured.
The semiconductor laser device of FIG. 17 is provided with gold electrodes 115, 116 on the upper face and the lower face of the laser chip 111 as shown in FIGS. 18A and 18B.
The gold electrode 115 on the upper face of the laser chip 111 is formed to have such a pattern that the width on the side of the light emitting end surface 111a is smaller than the width on the side of the light emitting end surface 111b. This pattern is for distinguishing the front-side light emitting end surface 111a of the laser chip 111 from the rear-side light emitting end surface 111b of the laser chip 111. The gold electrode 115 is formed to be smaller than the upper face of the laser chip 111, and the peripheral edge of the gold electrode 115 is not overlapped with the peripheral edge of the upper face of the laser chip 111.
The surface of the gold electrode 116 on the lower face of the laser chip 111 becomes a die bond face to cover the entire lower face of the laser chip 111. In other words, the gold electrode 116 is a so-called allover gold electrode. In this case, since the gold electrode 116 is the allover electrode, the gold electrode 116 and the Si thin film 123 come into contact at a point A as shown in FIG. 18B. As a result, as shown in FIGS. 19A to 19C, gold in the gold electrode 116 may diffuse toward the Si thin film 123 due to heating in deposition of Si. Diffusion areas 119, 120 of gold are shown in FIGS. 19B and 19C, respectively.
When the thickness of the Si thin film 123 is around 40 Å, in most cases, the gold diffusion area 120 extends to a light emitting point 124 as shown in FIG. 19C. Also, even when the thickness of the Si thin film 123 is around 20 Å as shown in FIG. 19C, the gold diffusion area 119 may extend to the light emitting point 124.
When the gold diffusion areas 119, 120 extend to the light emitting point 124 as described above, a maximum optical output value (so-called COD (Catastrophic Optical Damage) level) becomes about half of that or lower in the case of no gold diffusion. This causes a problem of considerably degraded reliability of the laser chip 111 as shown in FIGS. 20A and 20B.
As a solution of this problem, gold in the peripheral part of the gold electrode 116 is removed, so that the gold electrode 116 does not come into contact with the Si thin film 123 like the gold electrode 115 on the upper face of the laser chip 111. However, operations for removing gold in the peripheral part of the gold electrode 116 are complex to require time and cost.